1. Field of the Invention
The present invention relates to a manufacturing process of an electrode having a perforated current collector.
2. Description of the Related Arts
An electric storage device, such as a lithium ion battery or a lithium ion capacitor, is mounted on an electric vehicle or hybrid vehicle. When an electrode incorporated into an electric storage device is produced, an electrode slurry containing an active material is applied on a material of a current collector such as a metal foil. In general, the material of the current collector is conveyed in the horizontal direction so as to allow the material to pass through a drying furnace, whereby the electrode slurry is dried to form an electrode mixture layer.
An electric storage device has been proposed in which a metal lithium foil and a negative electrode are electrochemically in contact with each other in order to enhance an energy density of the electric storage device. In the electric storage device described above, lithium ions can be doped beforehand into the negative electrode. Thus, the potential of the negative electrode can be lowered, and the capacitance of the negative electrode can be increased. Therefore, the energy density of the electric storage device can be enhanced. Through-holes, through which the lithium ions pass, are formed on the current collector of each electrode in order to uniformly dope the lithium ions into the laminated plural negative electrodes (see JP-3485935).
When the electrode slurry is applied onto the current-collector material having the through-holes formed thereon, the electrode slurry might pass through the through-holes to reach the backside of the current-collector material. If the electrode slurry passes through the through-holes to reach the backside of the current-collector material, the electrode slurry might be deposited onto a guide roller that supports the current-collector material. In view of this, a manufacturing process has been proposed in which the current-collector material is lifted up in the vertical direction so as to apply the electrode slurry thereon. According to this manufacturing process, a guide roller is unnecessary during the process of lifting up the current-collector material, so that the deposition of the electrode slurry onto the guide roller can be prevented. Further, there has been proposed an electric storage device in which smaller through-holes are formed on the current-collector material in order to prevent the electrode slurry from passing through the through-holes to reach the backside of the current-collector material (see, for example, JP-A-2007-141897).
However, lifting up the current-collector material in the vertical direction entails the lowering of the conveying speed, which unfavorably lowers productivity of the electrode. Specifically, it is necessary to lift up the current-collector material in the vertical direction until the electrode slurry applied onto the current-collector material is dried. However, the current-collector material might be broken by its own weight, so that the limitation is imposed on the lifting height of the current-collector material. With the limitation on the lifting height, the limitation is also imposed on the height of the drying furnace used for drying the electrode slurry. It is necessary to decrease the conveying speed of the current-collector material in order to dry the electrode slurry with such a short drying furnace. As described above, lifting up the current-collector material in the vertical direction lowers the productivity of the electrode as well as increases the manufacturing cost.
When the through-holes are made small in order to prevent the electrode slurry from passing through the through-holes to reach the backside of the current-collector material, the electrode slurry can be applied while conveying the current-collector material in the horizontal direction. However, since the strength of the current-collector material provided with the through-holes is lower than the strength of the current-collector material having no through-holes formed thereon, the conveying speed of the current collector provided with the through-holes tends to lower. Even if the horizontal conveyance is made possible by the formation of the through-holes having smaller diameter, it is difficult to enhance productivity of the electrode compared to the case of the current-collector material having no through-holes formed thereon. Moreover, since the through-holes of the current collector are smaller, the moving speed of the lithium ions is lowered when the lithium ions are doped into the negative electrode. The lowering of the moving speed causes a long-term doping operation of the lithium ions into the negative electrode. The long-term doping operation deteriorates productivity of the electric storage device as well as increases manufacturing cost thereof.
There are a mechanical process such as pressing and a chemical process such as etching as a method for forming through-holes on the current-collector material. In terms of quality, the current-collector material is preferably subject to etching. However, when the current-collector material is subject to etching, a resist layer having a predetermined pattern has to be formed on the current-collector material. After the through-holes are formed by the etching, the resist layer has to be removed from the current-collector material. In this manner, many processes are required to perform the etching, which lowers productivity of the current collector as well as lowers productivity of the electrode.